It is well known in the art to provide an apparatus to maintain the tension in a power transmission belt which is adapted to be operated in an endless path. The tensioner maintains a constant tension on the belt to insure proper operation of the various driven devices. The tensioner is required due to changes in the belt length caused by belt wear, stretching, temperature variations, etc. Accordingly, in both automotive and non-automotive applications, various tensioning devices have been developed to compensate for these changes in belt length.
The present invention will be described incorporated into an endless belt drive system commonly used in automotive vehicles to power the various accessories. It is understood that the present invention is not to be limited to automotive accessory applications and is equally applicable to other automotive and non-automotive belt drive systems.
An automotive belt drive system normally receives power from the crankshaft and is used to power various accessories including alternators, water pumps, fans, air pumps, air conditioning compressors, power steering pumps and the like. In order to maintain proper tension on the belt as it drives these accessories, a spring loaded idler pulley is sometimes positioned within the system to constantly apply a load or tension on the belt to insure the belt will maintain engagement with the various drive pulleys and maintain a sufficient load or tension on the belt to insure the driving of the accessories.
Various designs of spring loaded idler pulleys have been developed. Some designs simply place tension on the belt, other designs incorporate braking mechanisms which resist movement of the pulley in a specific direction and still others incorporate a damping device which tends to isolate the movement of the idler pulley with respect to various natural frequencies encountered by the belt drive system.
Most of the prior art belt tensioning devices incorporate a spiral spring. The term spiral spring will be used throughout this disclosure to describe the present invention. It is to be understood that the term spiral spring includes clock wound springs, power springs or any other spring of similar construction. Normally, spiral springs are manufactured from a series of windings of a flat piece of spring wire steel. Most of these springs have multiple coils generally having unrolled lengths of between 10 and 50 inches depending on the requirements of the particular application. The spiral spring provides a change in torque which allows the idler pulley to maintain belt tension over a relatively large change in belt length or idler pulley position. One end of the spring is engaged with an arbor which is in turn secured to the engine block or other supporting structure. The opposite end of the spring is engaged with a housing which is in turn connected to the idler pulley. The spring reacts to forces within the system transmitted by the idler pulley to the housing to maintain constant tension on the belt.
While the prior art tensioning devices perform in an adequate manner, they are not without problems. In order to provide for a large amount of movement of the idler pulley, it is necessary to increase the length of the spiral spring. As the length of the spiral spring increases, the overall diameter of the spring increases, increasing the diameter of the housing which holds the spring. As the diameter of the spring housing increases it becomes more and more difficult to package the tensioning device. With automobiles becoming smaller, under hood space available for engines and their accessories is also being reduced. Therefore, it may be necessary to compromise the design of the prior art tensioning device in order to allow for the packaging of these devices. When the length of the spiral spring is kept to a minimum for packaging considerations, it is not uncommon for the spring to be overstressed as the tensioning device is forced into an overtravel condition during the assembly of the original vehicle or by the fitting of a new belt.
Accordingly, what is needed is a belt tensioning device which is equipped with a spiral spring which is capable of achieving an increased travel while not significantly adding to the packaging problems under the hood.